This project investigates the clinical manifestations and molecular genetic defects of Smith-Magenis Syndrome (SMS), a rare (1/15,000 1/25,000) clinically recognizable yet often under diagnosed multiple congenital anomaly/intellectual disability (MCA/ID) syndrome (OMIM #182290). The syndrome is characterized by a distinct pattern of minor craniofacial and skeletal anomalies, expressive speech/language delays, psychomotor and growth retardation, and a striking neurobehavioral phenotype that includes a circadian sleep disorder. The majority of cases (~90%) are due to de novo interstitial deletion of chromosome 17p11.2 that includes the RAI1 (retinoic acid induced 1) gene; however, heterozygous RAI1 mutations account for about 10% of cases. While clinical variability exists, haploinsufficiency of RAI1 (by deletion or mutation) is believed to be responsible for most of the common features that characterize the syndrome.